All modern hard disks are S.M.A.R.T. enabled. «SMART is a monitoring system for hard disks to detect and report on various indicators of reliability, in the hope of anticipating failures».
Most people never pay attention to the numbers, some people only pay attention to the templerature, and if something bad happens (a failure) then they give some attention to the error counters.
But here are two counters that I found very interesting: Power_On_Hours and Power_Cycle_Count.
A hard disk used in my desktop machine
Power_On_Hours 7566
Power_Cycle_Count 314
A hard disk used in a server machine
Power_On_Hours 45030
Power_Cycle_Count 28
To see this info about your hard disk, use «smartctl -a /dev/sda | grep Power».
smartctl is part of the smartmontools package in most linux distros.
This was triggered by the following tweet
Don't fear the penguin 
it became my favorite command instantly :-)
sniffing all my disk for and getting stats out of them :-)
thanks
ha!
just came up with an idea!
i can check if my disk was booted at another system as secondary with this command!
cool i can see if my disk was cloned!